In electrophotography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image), is formed on a surface of an electrophotographic element comprising at least an insulative photoconductive layer and an electrically conductive substrate. The electrostatic latent image is usually formed by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
In latent image formation the imagewise discharge is brought about by the radiation-induced creation of electron/hole pairs, which are generated by a material (often referred to as a charge-generation material) in the electrophotographic element in response to exposure to the imagewise actinic radiation. Depending upon the polarity of the initially uniform electrostatic potential and the type of materials included in the electrophotographic element, either the holes or the electrons that have been generated migrate toward the charged surface of the element in the exposed areas and thereby cause the imagewise discharge of the initial potential. What remains is a non-uniform potential constituting the electrostatic latent image.
Many electrophotographic elements currently in use are designed to be initially charged with a negative polarity. Such elements contain material which facilitates the migration of positive holes toward the negatively charged surface in imagewise exposed areas in order to cause imagewise discharge. Such material is often referred to as a hole-transport agent. In elements of that type a positively charged toner material is then used to develop the remaining imagewise unexposed portions of the negative polarity potential (i.e., the latent image) into a toner image. Because of the wide use of negatively charging elements, considerable numbers and types of positively charging toners have been fashioned and are available for use in electrographic developers. Conversely, relatively few high quality negatively charging toners are available.
However, for some applications of electrophotography it is more desirable to be able to develop the surface areas of the element that have been imagewise exposed to actinic radiation, rather than those that remain imagewise unexposed. For example, in laser printing of alphanumeric characters it is more desirable to be able to expose the relatively small percentage of surface area that will actually be developed to form visible alphanumeric toner images, rather than waste energy exposing the relatively large percentage of surface area that will constitute undeveloped background portions of the final image. In order to accomplish this while still employing widely available high quality positively charging toners, it is necessary to use an electrophotographic element that is designed to be positively charged. Thus, positive toner can then be used to develop the exposed surface areas (which will have relatively negative electrostatic potential after exposure and discharge, compared to the unexposed areas, where the initial positive potential will remain).
An electrophotographic element designed to be initially positively charged should, however, contain an adequate electron-transport agent (i.e., a material which adequately facilitates the migration of photo-generated electrons toward the positively charged insulative element surface). Unfortunately (and analogous to the situation with positive and negative toners), many materials having good hole-transport properties have been fashioned for use in electrophotographic elements, but relatively few materials are known to provide good electron-transport properties in electrophotographic elements.
A number of chemical compounds having electron-transport properties are described, for example, in U.S. Pat. Nos. 4,175,960; 4,514,481; 4,474,865; 4,559,287; 4,606,861; and 4,609,602. However, many prior art compounds have one or more drawbacks.
Some prior art electron-transport agents do not perform the electron-transporting function very well, especially under certain conditions or when included in certain types of electrophotographic elements. Also, some cause an undesirably high rate of discharge of the electrophotographic element before it is exposed to actinic radiation (often referred to as high dark decay).
Furthermore, some prior art electron-transport compounds are not soluble or dispersible or have relatively limited solubility or dispersibility in coating solvents of choice or in some polymeric binders desired to be used in charge-transport layers, such that attempts to include sufficient amounts of such electron-transport agents in electrophotographic elements result in some crystallization of the electron-transport agents, which in turn causes problems such as undesirable levels of dark decay and such as unwanted scatter or absorption of actinic radiation intended to pass undisturbed through the charge-transport layer to a radiation-sensitive portion of the element.
Even when sufficient amounts of electron-transport agent for adequate performance can be initially compatibly incorporated in an electrophotographic element, problems can arise thereafter during use of the element. For example, U.S. Pat. No. 4,514,481 describes a number of specific electron-transport agents (e.g., 4-dicyanomethylene-2,6-diphenyl-4H-thiopyran-1,1-dioxide) and illustrates incorporating them in polymeric binder layers of electrophotographic elements at a concentration of 30% by weight (based on total weight of the agent and the binder) for good performance. However, in fact, the upper limit of compatibility (solubility or homogeneous dispersibility) of compounds such as 4-dicyanomethylene-2,6-diphenyl-4H-thiopyran-1,1-dioxide in many polymeric binders is about 35% by weight. At such concentration the compounds are on the edge of incompatibility. At elevated temperatures, such as the element can encounter during normal use in a copier, the compound can more easily migrate within the binder and tend to form crystalline agglomerates that cause problems such as noted above.
Even if such a problem does not occur, if it is desired to increase the concentration of such an electron-transport agent beyond 35% by weight, there is no leeway to do so. For example, it is well known in the art that increasing the concentration of an electron-transport agent in a polymeric layer, without phase-separation, increases the electron-transport mobility of the layer; i.e., photogenerated electrons will move through the layer at a higher velocity and will traverse the layer in a shorter period of time. Such increased mobility enables use of an element, for example, in a high speed copier employing high-intensity, short-duration imagewise exposure (commonly also referred to as flash exposure), wherein the time it will take for the element to properly discharge, and, thus, the length of the period needed between the end of the exposure step and the beginning of the toner image development step, is determined by the level of electron-mobility within the element. The higher the mobility is, the shorter is the necessary waiting period between exposure and development, and the greater is the number of copies that can be made in a given amount of time.
Thus, there is a continuing need for electrophotographic elements containing new chemical compounds that will serve well as electron-transport agents in the elements without imparting undesirably high dark decay characteristics thereto and that will also exhibit improved solubility or dispersibility in coating solvents of choice and improved compatibility with polymeric film-forming binders of choice.
The present invention meets this need.